<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
    您的浏览器不支持 HTML5 canvas 标签。
</canvas>
<script>
    var c = document.getElementById("myCanvas");
    var ctx = c.getContext("2d");

    ctx.fillStyle = "red";
    ctx.fillRect(20, 20, 75, 50);
    ctx.globalCompositeOperation = "source-over";
    ctx.fillStyle = "blue";
    ctx.fillRect(50, 50, 75, 50);

    ctx.fillStyle = "red";
    ctx.fillRect(150, 20, 75, 50);
    ctx.globalCompositeOperation = "destination-over";
    ctx.fillStyle = "blue";
    ctx.fillRect(180, 50, 75, 50);
</script>
<style>
    body {
        margin: 0;
    }
</style>